A third nuclear core was already ready to be shipped to the front, which would've been dropped on August 19th. 2 0.5 uranium or and 3 plutonium bombs could've been produced per month. See here by /u/restricteddata, who also uploaded the transcript of a meeting between General Hull and Colonel Seaman to his blog. It was estimated three definitely ready in each of September and October, with a possibility of squeezing a fourth into September, for a total of six to seven bombs over the two months.